Bulger had just been moved to USP Hazelton, a high-security prison with an adjacent minimum security satellite camp in Bruceton Mills.

The FBI is investigating the death and law enforcement sources tell CBS News that it appears Bulger was severely beaten by one or more of his fellow inmates shortly after his arrival at the federal prison in West Virginia on Monday.

In a statement Tuesday afternoon, the prison would only say Bulger was “found unresponsive” at 8:20 a.m. and that “life-saving measures were initiated immediately by responding staff.” He was pronounced dead by the local medical examiner.

“The Federal Bureau of Investigation was notified and an investigation has been initiated. No staff or other inmates were injured, and at no time was the public in danger,” the statement read.

According to the Boston Globe, a “fellow inmate with Mafia ties” was being investigated for Bulger’s death.

U.S. Attorney for Massachusetts Andrew Lelling said his office had received word of Bulger’s death and “our thoughts are with his victims and their families.”

Bulger had recently been moved from a prison in Florida to a transfer facility in Oklahoma City. Bureau of Prisons officials and his attorney declined last week to comment on why he was being moved.

Whitey Bulger as he was arrested by the FBI, June 22, 2011. (Photo courtesy: 60 Minutes-CBS News)

Bulger was serving a life sentence after being convicted in 2013 of a litany of crimes, including participating in 11 murders.

“I’d like to open up a champagne bottle and celebrate,” Patricia Donahue told WBZ-TV Tuesday. Bulger was convicted of killing her husband Michael in 1982.

#WhiteyBulger and his thugs ruthlessly lured my dad’s friend Eddie Connors to a phone booth and shot him because he refused to pay protection money for his bar. Justice has been done, a little late IMO.

He was one of the FBI’s most wanted fugitives for 16 years until his 2011 arrest in Santa Monica, California.

Attorney J.W. Carney Jr. said he was proud to represent Bulger in his 2013 trial. “He was sentenced to life in prison, but as a result of decisions made by the Federal Bureau of Prisons, that sentence has been changed to the death penalty,” Carney said in a statement.

Bulger was born in South Boston during the Depression and got the nickname “Whitey” growing up for his blonde hair. A high school dropout, Bulger began a life of crime that eventually led to prison stays at Alcatraz and Leavenworth. But Whitey made it back to Boston where he slowly resumed a career in crime that led him to Somerville’s Winter Hill Gang, a rag-tag band of thieves that dabbled in stolen goods, bookmaking, extortion and drugs.

Bulger would eventually become an informant for the FBI, something he always denied, giving up information on his so-called friends that put them in jail while Whitey rose in the ranks of Boston’s organized crime. Even the city’s most powerful crime family, the Angiulos, would go to prison, based on information Bulger gave to the FBI.

Watch: Whitey Bulger Obituary

FBI agents overseeing Bulger as an informant later testified that Bulger could commit any crime, except murder, in exchange for information. With that, it’s believed Bulger made millions of dollars selling drugs and extorting drug dealers, all while staying out of jail while he ratted out other mobsters to his FBI handlers.

But by 1995, even Bulger’s friends in high places couldn’t keep him from an indictment – this time for obstruction of justice, racketeering, drug dealing and extortion. On the eve of the indictment, Bulger was tipped off by his FBI handler John Connolly.

He went on the run with his girlfriend Catherine Greig, ending up in Santa Monica, California.

While he was on the run, Bulger was indicted for more than a dozen murders that prosecutors say happened during his reign of terror in Boston. Investigators say Bulger stashed millions in bank accounts around the country before he fled.

James “Whitey” Bulger (Photo from U.S. Marshals Service)

During 16 years on the run, Bulger was on the FBI’s 10 Most Wanted list. He was finally captured in June 2011 in Santa Monica, Calif., where he had been living in a rent-controlled apartment near the beach with Greig. She was sentenced to eight years in prison for helping Bulger evade the law. She is currently being held in Minnesota and is due to be released in 2020.

“I think that at the end of the day the truth and justice caught up with him,” said Carmen Ortiz, former U.S. Attorney for Massachusetts. “I hope this is the end of a really sad chapter in Boston’s history.”